The endocrine system is a physiological control system that secretes hormones (chemical signal/messengers) that circulate in the bloodstream to affect distant target or receptor cells of specific organs. Endocrine glands secrete hormones immediately into the blood or interstitial fluid, without storage of the chemical. Hormones act as “signalers” or “messengers,” which interpret these chemical messengers and respond physiologically by increasing or decreasing metabolic rate, dilating or constricting blood vessel, dilating airway passages, or releasing other hormones. The response of the target tissue is commonly called signal transduction when one signal or stimulus is turned to another signal or process. Sometimes a small stimulus causes a larger response known as a ‘signaling cascade’ or ‘second messenger pathway.’
The endocrine system is regulated by negative feedback, which maintains an equilibrium or homeostatis that works like a thermostat. The process keeps the body within normal limits or a setpoint, but lets it get to extremes in human performance or under conditions of environmental, physical or mental stress. The word ‘negative’ refers to a control mechanism so that if something is getting toward abnormal, the endocrine system hopefully uses hormonal control to bring things back to the setpoint in an opposite or ‘negative’ direction from the conditions that are causing abnormality. If there is too much stress or body damage in a situation, the body’s response can be a positive feedback, as in the case of shock, and the body can go to an irreversible physiological state that can cause death.
There are four types of hormones:
1. Amino-acid derived hormones (modified amino acids, most are water soluble)
2. Polypeptide Hormones and Protein Hormones (most are water soluble). Polypeptide hormones are characterized as a chain of less than 100 amino acids. Protein hormones are chains of more than 100 amino acids and often included glucose or other carbohydrates to form glycoproteins.
3. Steroid Hormones (lipid soluble) are derived from cholesterol and are characterized by four carbohydrate rings.
4. Eicosanoids are lipid messengers that are dreived from fatty chains of phospholipids found in plama membranes.
Lipid Soluble Hormones are steroid hormones and thyroid gland hormones that reach the cell membranes of their target cells, diffuse across the membrane, and bind to a receptor protein that activates a DNA segment that turns on specific genes. These genes control specific cellular and physiological activity with their transcription and translation of messenger RNA and enzyme production.
Water Soluble Hormones (most amino acid hormones, polypeptide hormones, and protein hormones) target receptor cells on the plasma membrane of cells, which stimulates production of one of two second messengers (cyclicAMP– cAMP or Inositol triphosphate — IP3).